The French Foreign Minister Jean-Marc Ayrault said that samples taken from the attack on the rebel-held town of Khan Sheikhoun matched those from a previous incident.

“We have definite sources that the procedure used to make the Sarin sampled is typical of the methods developed in Syrian laboratories,” he said. “This method bears the signature of the regime, and that is what has allowed us to establish its responsibility in this attack.”

French laboratories had stored samples taken from other chemical attacks in Syria and so were able to compare them, he said.

A tweet posted by the French Foreign Ministry said: “There’s no doubt that Sarin was used. There is also no doubt about the responsibility of the Syrian regime.”

The attack has been widely blamed by Western powers on the Syrian government, which is supposed to have given up its chemical weapon stockpile in 2013 following an attack in the Ghouta area of Damascus that activists say killed 1,400 people.

Kremlin spokesman Dmitry Peskov said Wednesday that Russia would not change its position regarding the Khan Sheikhoun attack in light of the French assessment.

“The Kremlin and President Putin still believe that conducting an impartial international investigation is the only way to find out the truth,” state-run TASS quoted Peskov as saying.

A Syrian man collects samples from the site of a suspected toxic gas attack in Khan Sheikhoun on April 5.

The French Foreign Ministry said its independent investigation, declassified so it could be shared with the world, supported “with certainty” the conclusions also reached by the United States, United Kingdom, Turkey and the OPCW.

Analysis by French experts of samples from the April 4 attack site and the blood of one of the victims confirmed the use of Sarin, its report said. Those samples were compared with samples from an attack on the northern Syrian town of Saraqeb in April 2013, in which three grenades containing Sarin were dropped by a helicopter, one of which failed to explode, it said. According to the French army, only the Syrian regime had helicopters so it had to be behind the attack.

Syrians bury the bodies of victims of the attack in Khan Sheikhoun, in Idlib province, on April 5, 2017.

Scientists established the presence of the same chemical compounds in samples taken from Saraqeb in 2013 and from Khan Sheikhoun, the French Foreign Ministry said. “The Sarin present in the weapons used on April 4 was produced according to the same manufacturing method as that used in the Sarin attack carried out by the Syrian regime in Saraqeb.”

The report also cited the French military’s assessment that a warplane had been deployed from the Syrian regime’s Shayrat airbase on the morning of April 4 and had carried out up to six airstrikes in the Khan Sheikhoun area. “Only the regime has such air assets,” said Ayrault.

“The French intelligence services believe that only Bashar al-Assad and some of the most influential members of his entourage are empowered to give the order to use chemical weapons,” the report added.

The report also describes the claim that rebel forces in the area had Sarin as “not credible.”

It casts doubt on the Syrian regime’s promised destruction of its chemical stockpile, saying that French intelligence services believe “important doubts remain about the accuracy, completeness and sincerity of the dismantling of the Syrian chemical arsenal.”